From List[Either[A, B]] to Either[A, List[B]]

ComposingFutureListAndEither.scala

import scala.language.postfixOps

import scala.concurrent.ExecutionContext
import scala.concurrent.Future

import cats.data.EitherT
import cats.instances.list._
import cats.instances.future._
import cats.syntax.traverse._

implicit val ec: ExecutionContext = scala.concurrent.ExecutionContext.Implicits.global

def getCommitsWithAuthor(): Future[Either[Error, List[CommitAndAuthor]]] =
  (for {
    commits <- EitherT(getCommits())
    result <- commits.map { commit =>
      EitherT(getAuthor(commit.id)) map { author => CommitAndAuthor(commit, author) }
    }.sequenceU
  } yield result).value

final case class Commit(id: String)
final case class Author(id: String)
final case class CommitAndAuthor(commit: Commit, author: Author)

sealed trait Error

def getCommits(): Future[Either[Error, List[Commit]]] =
  Future.successful(
    Right(
      List(
        Commit("abcdef"),
        Commit("ghijk")
      )
    )
  )

def getAuthor(commitId: String): Future[Either[Error, Author]] =
  Future.successful(
    Right(
      Author("me")
    )
  )

import scala.concurrent.Await
import scala.concurrent.duration._

println(Await.result(getCommitsWithAuthor(), 10000 millis))